Composite materials are often used in the construction of aerospace and other structures because of their high strength-to-weight ratios. Many aircraft, for example, utilize composite materials for both internal airframe components and external skin panels. Composite materials are often used in the form of a panel that can include a core material sandwiched between two opposing face sheets. The face sheets can include one or more plies of a composite fabric material, and the core can include a lightweight material such as a foam or honeycomb material. The resulting combination provides a lightweight panel having relatively high strength and stiffness. The panel can be fastened or otherwise attached to frames or other parts as required to complete a particular assembly.
FIGS. 1A–C are cross-sectional views of portions of composite panel assemblies 100a–c having composite panels 101a–c configured in accordance with the prior art. Referring to FIG. 1A, the prior art composite panel 101a includes a first face sheet 102a, a second face sheet 104a offset from the first face sheet 102a, and a core 106a sandwiched between and bonded to the first and second face sheets 102a and 104a. A fastener 120a extends through the composite panel 101a and attaches a part 110 to the composite panel 101a. 
One feature of the prior art composite panel 101a is that the fastener 120a extends through holes in both the first and second face sheets 102a and 104a. A disadvantage of this feature is that water or other fluids can leak through the fastener hole if one side of the composite panel 101a is exposed to these fluids. This shortcoming makes the composite panel 101a a poor choice in those applications where a leak-proof panel is desired. In addition, fluids leaking into the composite panel 101a can compromise the structural integrity of the core 106a and/or the bond between the core 106a and the first and second face sheets 102a and 104a. 
Another feature of the composite panel 101a is that it has relatively low compression strength through the panel thickness because the core 106a has a tendency to crush under compression load. A disadvantage of this feature is that the fastener 120a cannot develop sufficient pre-load to adequately clamp the part 110 to the panel 101a. To overcome this problem, potting (not shown) can be injected into the composite panel 101a and cured to provide compression strength around the fastener 120a, preventing the core 106a from crushing under fastener pre-load. One downside of this solution, however, is that additional manufacturing effort is required to install the potting. Further, while this solution may reduce the tendency of the core 106a to crush, it does not prevent fluid leakage.
Referring next to FIG. 1B, the prior art composite panel 101b includes a first face sheet 102b, a second face sheet 104b, and core portions 106b sandwiched between the first and second face sheets 102b and 104b. The core portions 106b taper inwardly toward a coreless region 103b where the second face sheet 104b is bonded directly to the first face sheet 102b. A fastener 120b extends through the first and second face sheets 102b and 104b in the coreless region 103b and attaches the part 110 to the composite panel 101b. 
One feature of the prior art composite panel 101b is the lack of panel thickness in the coreless region 103b. A disadvantage of this feature is that the ability of the composite panel 101b to withstand large bending moments is substantially reduced. Another disadvantage of the coreless region 103b is that tapering the edges of the core portions 106b to form this region requires additional manufacturing expense. A further disadvantage is the fluid leakage problem described above with reference to FIG. 1A.
Referring now to FIG. 1C, the prior art composite panel 101c includes a first face sheet 102c, a second face sheet 104c, and core portions 106c. Potting 107 has been injected into the space between the core portions 106c and recessed to provide room for a blind fastener 120c. The blind fastener 120c extends through the part 110 and the second face sheet 104c and attaches the part 110 to the composite panel 101c. The blind fastener 120c is referred to as a “blind fastener” because it can be fully installed through the second face sheet 104c with access from only one side of the composite panel 101c. 
One feature of the prior art composite panel 101c is that the blind fastener 120c extends only through the second face sheet 104c. While this feature may prevent the fluid leakage problems described above, it has the disadvantage of providing a relatively weak attachment for the part 110. Applying significant loads to the part 110 can cause the second face sheet 104c to deflect and pull away from the potting 107 and/or the core portions 106c in the region adjacent to the blind fastener 120c. Another disadvantage of this approach is that additional manufacturing effort is required to install and recess the potting 107.